It seems that it’s easy to discern the difficulty mode of a boss kill screenshot, just by looking at it.
Mythic: Everyone lined up, various angles taken.
Heroic: Most people there, a few random toys popped
Normal: Half the raid gone as soon as it’s dead, others sprinting for the portal, and the insta pings of people leaving discord.
So, yeah, this was our Normal G’huun kill…*tries to remember the date*…